For intrusion detection and more specifically to detect penetration of strong rooms, safes or vaults, it is oftentimes desirable to detect not only activity adjacent the vault indicating the presence of an intruder but also to detect the initial attempts at penetration of the vault. Intrusion detection usually is accomplished with the aid of air-conducted acoustic waves in which active ultrasonic detectors or passive acoustic detectors are utilized in the surveillance of a protected area. Moreover, some systems utilize microwave energy projected into the protected area, variations of which are detected to determine a alarm condition.
It will be appreciated that once an intruder has penetrated a protected area such as the vault, it is oftentimes too late for an alarm indication to be of real use because of the accessability of the valuables being protected. It is therefore necessary, in a wide variety of cases to provide "early warning" of a vault penetration so that appropriate early countermeasures may be taken.
While vault activity detectors have been utilized in the past, it is sometimes with difficulty that ordinary sounds and vibrations can be distinguished from those which would indicate an unauthorized penetration or attempted penetration of the vault, safe or strong room. Thus sensing vault activity alone may result in an unacceptable high false alarm rate.
As illustrated in U.S. Pat. Nos. 3,801,978; 4,103,293; 3,725,888; and 4,121,182, prior art detection systems have utilized multiple and differing sensors for sensing air-conducted signals. These sensors have been utilized singly or in combination to reduce the false alarm rate for the systems in which they are employed but do not deal with the combination of structurally-conducted signals and air-conducted signals for enhanced detected. As a result, the systems illustrated by the aforementioned patents do not necessarily provide a fail-safe "early warning" system usable for vault security.